Until recently, it was not possible to produce sticky polymers such as ethylene-propylene and ethylene-propylene-diene rubbers (EPRs and EPDMs), collectively referred to as ethylene-propylene rubbers, in a gas-phase polymerization under commercially attractive conditions.
U.S. Pat. No. 4,994,534 made commercial production feasible by teaching that sticky polymers, such as ethylene-propylene rubbers, can be produced in a gas-phase polymerization process using an inert particulate material (or fluidization aid) at temperatures at or above the polymer softening or sticking temperature. The use of an inert particulate material renders a sticky polymer such as ethylene-propylene rubber non-sticky so that it can be fluidized in gas-phase operation. Sufficient quantities of inert particulate material (carbon black and other carbon materials, silica, clays, talc, and other materials which are inert under reaction conditions) are added to the reactor to permit production of sticky polymers having an outer shell comprised mostly of inert particulate material. It is believed that in the polymer particle so made, the outer shell comprised mostly of inert particulate material renders the core comprised mostly of polymer, and hence the whole of the particle non-sticky. Levels of fluidization aids ranging from 0.3 to 80 weight percent, based on the weight of the polymer plus fluidization aid, and having a mean particle size from about 0.01 to about 10 microns have been effective in such polymerizations. It should be pointed out that the neat polymer (absence of inert particulate material) is sticky, but after it is contacted with fluidization aid the polymer does not behave as a sticky polymer, that is, it is non-sticky and flowable.
However, while making gas-phase production of sticky polymers commercially attractive, inert particulate materials introduce their own problems to the polymerization process. These polymerizations of elastomers are significantly different from and much more difficult to conduct than gas-phase polymerizations to produce polyethylene and polypropylene polymers.
For example, the inert particulate materials can act as fines which can travel through the reaction system (distributor plate, cycle piping, heat exchanger(s), compressors, etc.) and foul it. The use of inert particulate materials such as silica and clay can result in the build-up of static electricity which negatively affects operability of the reaction system. Also, inadequate or improperly dispersed inert particulate material can result in formation of skins and sheets of polymer in the expanded section of the fluidized-bed reactor. The polymer skins or sheets can fall into the fluidized bed and appear as agglomerates in the elastomeric product as well as cause distributor plate plugging. Skin thermocouples in the reactor can register temperature irregularities, thereby indicating the presence of skins. But, if in response to these thermocouple readings, the operator increases the level of inert particulate material, reactor shutdown could result because of too much inert particulate material in the reactor. These phenomena associated with the use of inert particulate materials when polymerizing sticky polymers such as ethylene propylene polymers are particularly troublesome.
Accordingly, it is a principal object of the present invention to continuously produce ethylene propylene rubber containing inert particulate material in a gas-phase, fluidized-bed reactor over long periods of time with good reactor operability. It is another object to produce ethylene-propylene rubber which is substantially free of agglomerates. A further object of the invention is to reduce the required amount of inert particulate material required for ethylene-propylene rubber production while achieving the above objectives. These and other objectives will be apparent from the following description of the invention.